Lock modes can be specified by means of passing a LockModeType
argument to one of the EntityManager methods that take locks
(lock, find, or refresh) or
to the Query.setLockMode() or
TypedQuery.setLockMode() method.

Lock modes can be used to specify either optimistic or pessimistic locks.

Optimistic locks are specified using LockModeType.OPTIMISTIC and LockModeType.OPTIMISTIC_FORCE_INCREMENT. The lock mode type
values LockModeType.READ and
LockModeType.WRITE are
synonyms of OPTIMISTIC and
OPTIMISTIC_FORCE_INCREMENT respectively. The latter
are to be preferred for new applications.

The semantics of requesting locks of type
LockModeType.OPTIMISTIC and
LockModeType.OPTIMISTIC_FORCE_INCREMENT are the
following.

If transaction T1 calls for a lock of type
LockModeType.OPTIMISTIC on a versioned object,
the entity manager must ensure that neither of the following
phenomena can occur:

P1 (Dirty read): Transaction T1 modifies a row.
Another transaction T2 then reads that row and obtains
the modified value, before T1 has committed or rolled back.
Transaction T2 eventually commits successfully; it does not
matter whether T1 commits or rolls back and whether it does
so before or after T2 commits.

P2 (Non-repeatable read): Transaction T1 reads a row.
Another transaction T2 then modifies or deletes that row,
before T1 has committed. Both transactions eventually commit
successfully.

Lock modes must always prevent the phenomena P1 and P2.

In addition, calling a lock of type
LockModeType.OPTIMISTIC_FORCE_INCREMENT on a versioned object,
will also force an update (increment) to the entity's version
column.

The persistence implementation is not required to support
the use of optimistic lock modes on non-versioned objects. When it
cannot support a such lock call, it must throw the PersistenceException.

The lock modes LockModeType.PESSIMISTIC_READ, LockModeType.PESSIMISTIC_WRITE, and
LockModeType.PESSIMISTIC_FORCE_INCREMENT are used to immediately
obtain long-term database locks.

The semantics of requesting locks of type
LockModeType.PESSIMISTIC_READ, LockModeType.PESSIMISTIC_WRITE, and
LockModeType.PESSIMISTIC_FORCE_INCREMENT are the following.

If transaction T1 calls for a lock of type
LockModeType.PESSIMISTIC_READ or
LockModeType.PESSIMISTIC_WRITE on an object, the entity
manager must ensure that neither of the following phenomena can
occur:

P1 (Dirty read): Transaction T1 modifies a
row. Another transaction T2 then reads that row and obtains the
modified value, before T1 has committed or rolled back.

P2 (Non-repeatable read): Transaction T1 reads a row. Another
transaction T2 then modifies or deletes that row, before T1 has
committed or rolled back.

A lock with LockModeType.PESSIMISTIC_WRITE can be obtained on
an entity instance to force serialization among transactions
attempting to update the entity data. A lock with
LockModeType.PESSIMISTIC_READ can be used to query data using
repeatable-read semantics without the need to reread the data at
the end of the transaction to obtain a lock, and without blocking
other transactions reading the data. A lock with
LockModeType.PESSIMISTIC_WRITE can be used when querying data and
there is a high likelihood of deadlock or update failure among
concurrent updating transactions.

The persistence implementation must support use of locks of type
LockModeType.PESSIMISTIC_READLockModeType.PESSIMISTIC_WRITE on a non-versioned entity as well as
on a versioned entity.

When the lock cannot be obtained, and the database locking
failure results in transaction-level rollback, the provider must
throw the PessimisticLockException and ensure that the JTA
transaction or EntityTransaction has been marked for rollback.

When the lock cannot be obtained, and the database locking
failure results in only statement-level rollback, the provider must
throw the LockTimeoutException (and must not mark the transaction
for rollback).

valueOf

Returns the enum constant of this type with the specified name.
The string must match exactly an identifier used to declare an
enum constant in this type. (Extraneous whitespace characters are
not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

IllegalArgumentException - if this enum type has no constant
with the specified name